The CCITT is a permanent, advisory body of the International Telecommunication Union
(ITU), which is responsible, among other things, for international recommendations and standardisation in telecommunications. In this international advisory committee for telegraph services, telephone services and telecommunication services, the telecommunications administrations and manufacturers drew up the CCITT recommendations, which have been called ITU recommendations since the structural reform. They serve as guidelines for the procurement of equipment, for telecommunications services and for the approval of terminal equipment wishing to use these services. Generally known are the recommendations of the V. and X. series for interfaces and since 1984 the I. series for ISDN
. The results of the CCITT work were published every four years in books of different colours. There is the Red Book from 1960, followed by the Blue, White, Green, Orange, Yellow, White books.